625 research outputs found
Broad-band study with Suzaku of the magnetar class
Broad-band (0.8-70 keV) spectra of the persistent X-ray emission from 9
magnetars were obtained with Suzaku, including 3 objects in apparent outburst.
The soft X-ray component was detected from all of them, with a typical
blackbody temperature of kT ~ 0.5 keV, while the hard-tail component,
dominating above ~10 keV, was detected at ~1 mCrab intensity from 7 of them.
Therefore, the spectrum composed of a soft emission and a hard-tail component
may be considered to be a common property of magnetars, both in their active
and quiescent states. Wide-band spectral analyses revealed that the hard-tail
component has a 1-60 keV flux, Fh, comparable to or even higher than that
carried by the 1-60 keV soft component, Fs. The hardness ratio of these
objects, defined as xi=Fh/Fs, was found to be tightly anti-correlated with
their characteristic age tau as xi=(3.3+/-0.3)x(tau/1 kyr)^(-0.67+/-0.04) with
a correlation coefficient of -0.989, over the range from xi~10 to xi~0.1.
Magnetars in outburst states were found to lie on the same correlation as
relatively quiescent ones. This hardness ratio is also positively correlated
with their surface magnetic fields with a correlation coefficient of 0.873. In
addition, the hard-tail component becomes harder towards sources with older
characteristic ages, with the photon index changing from ~1.7 to ~0.4.Comment: 8 pages, 4 figures, accepted for publication in The Astrophysical
Journal Letter
Spectral Comparison of Weak Short Bursts to the Persistent X-rays from the Magnetar 1E 1547.0-5408 in its 2009 Outburst
In January 2009, the 2.1-sec anomalous X-ray pulsar 1E 1547.0-5408 evoked
intense burst activity. A follow-up Suzaku observation on January 28 recorded
enhanced persistent emission both in soft and hard X-rays (Enoto et al. 2010b).
Through re-analysis of the same Suzaku data, 18 short bursts were identified in
the X-ray events recorded by the Hard X-ray Detector (HXD) and the X-ray
Imaging Spectrometer (XIS). Their spectral peaks appear in the HXD-PIN band,
and their 10-70 keV X-ray fluences range from ~2e-9 erg cm-2 to 1e-7 erg cm-2.
Thus, the 18 events define a significantly weaker burst sample than was ever
obtained, ~1e-8-1e-4 erg cm-2. In the ~0.8 to ~300 keV band, the spectra of the
three brightest bursts can be represented successfully by a two-blackbody
model, or a few alternative ones. A spectrum constructed by stacking 13 weaker
short bursts with fluences in the range (0.2-2)e-8 erg s-1 is less curved, and
its ratio to the persistent emission spectrum becomes constant at ~170 above ~8
keV. As a result, the two-blackbody model was able to reproduce the stacked
weaker-burst spectrum only after adding a power-law model, of which the photon
index is fixed at 1.54 as measured is the persistent spectrum. These results
imply a possibility that the spectrum composition employing an optically-thick
component and a hard power-law component can describe wide-band spectra of both
the persistent and weak-burst emissions, despite a difference of their fluxes
by two orders of magnitude. Based on the spectral similarity, a possible
connection between the unresolved short bursts and the persistent emission is
discussed.Comment: 21 pages, 18 figures and 3 tables. Accepted for publication in
Monthly Notices of the Royal Astronomical Society Main Journa
Suzaku View of the Neutron Star in the Dipping Source 4U 182237
The dipping X-ray source 4U 182237 was observed by Suzaku on 2006 Octrober
20 for a net exposure of 37 ks. The source was detected with the XIS at a 1-10
keV flux of 5.5 erg cm s, and with the HXD
(HXD-PIN) at a 10-50 keV flux of 8.9 erg cm s.
With HXD-PIN, the pulsation was detected at a barycentric period of 0.592437 s,
and its change rate was reconfirmed as 10 s s. The
1-50 keV spectra of 4U 1822-37 were found to be very similar to those of Her
X-1 in the slopes, cutoff and iron lines. Three iron lines (Fe K, Fe
XXV, and Fe XXVI) were detected, on top of a 1-50 keV continuum that is
described by an NPEX model plus a soft blackbody. In addition, a cyclotron
resonance scattering feature was detected significantly ( confidence),
at an energy of 332 keV with a depth of 0.4. Therefore,
the neutron star in this source is concluded to have a strong magnetic field of
2.8 G. Further assuming that the source has a relatively high
intrinsic luminosity of several times 10 erg s, its spectral and
timing properties are consistently explained
Soft and Hard X-Ray Emissions from the Anomalous X-ray Pulsar 4U 0142+61 Observed with Suzaku
The anomalous X-ray pulsar 4U 0142+61 was observed with Suzaku on 2007 August
15 for a net exposure of -100 ks, and was detected in a 0.4 to ~70 keV energy
band. The intrinsic pulse period was determined as 8.68878 \pm 0.00005 s, in
agreement with an extrapolation from previous measurements. The broadband
Suzaku spectra enabled a first simultaneous and accurate measurement of the
soft and hard components of this object by a single satellite. The former can
be reproduced by two blackbodies, or slightly better by a resonant cyclotron
scattering model. The hard component can be approximated by a power-law of
photon index \Gamma h ~0.9 when the soft component is represented by the
resonant cyclotron scattering model, and its high-energy cutoff is constrained
as >180 keV. Assuming an isotropic emission at a distance of 3.6 kpc, the
unabsorbed 1-10 keV and 10-70 keV luminosities of the soft and hard components
are calculated as 2.8e+35 erg s^{-1} and 6.8e+34 erg s^{-1}, respectively.
Their sum becomes ~10^3 times as large as the estimated spin-down luminosity.
On a time scale of 30 ks, the hard component exhibited evidence of variations
either in its normalization or pulse shape.Comment: 24 pages, 8 figures, accepted for publication in Publications of the
Astronomical Society of Japa
On the X-ray emission mechanisms of the persistent source and very low-fluence bursts of SGR J0501+4516
We present here a detailed spectral study of the X-ray emission of the persistent source and the low-fluence bursts of SGR J0501+4516 observed during a deep XMM-Newton observation near the peak of its 2008 outburst. For the persistent emission we employ a physically motivated continuum emission model and spectroscopically determine important source properties; such as, the surface magnetic field strength and the magnetospheric scattering optical depth. We find that the magnetar surface temperature near the peak of its activity is 0.38 keV, corresponding to an emission area of 131 km^2 at a distance of 2 kpc. The surface magnetic field strength determined spectroscopically, B=2.2E14 G, is consistent with the dipole field strength inferred from the source spin and spin down rate. We fit the stacked spectra of 129 very faint bursts with a modified blackbody model and find a temperature of 1.16 keV, corresponding to an emission area of 93 km^2. We also find an evidence for cooling during the burst decay phase
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